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More evidence for < 2,300

Last week's review article was a look at the state of the research into dietary sodium and its association with blood pressure, heart disease and death from all causes. As the authors noted, most of the research on salt in the diet are observational studies: they can show association, but not causality. That said, some observational studies can be more convincing than others.

An ongoing study in Spain, known as PREDIMED, has been evaluating the effects of three diets on various health parameters: a Mediterranean-style diet supplemented with extra-virgin olive oil, a Mediterranean-style diet supplemented with nuts, and a low-fat diet. On an annual basis the nearly 7,500 Spaniards at high risk for cardiovascular disease completed a food-frequency questionnaire with the assistance of a dietitian and responded to a questionnaire about such lifestyle variables as physical activity, education, and medications. Their blood pressures, height, weight, and waist circumference were also measured.

With the information gathered in the dietary questionnaires, the authors were able to estimate the amount of sodium in each participant's diet and classify them into 4 increasing levels of sodium intake, ranging from less than 1,500mg of sodium per day to over 3,400mg/day (Am J Clin Nutr 2015;101:440-8). With the multiple years of data, they were also able to see which participants increased their sodium intake to over 2,300mg/day as well as those who decreased their intake to below that point.

After controlling for a truly impressive number of variables, including Body Mass Index; age; smoking status: which diet they were assigned to; family medical history; caloric intake (as well as whether that intake increased or decreased); and amounts of dietary fats, fiber, calcium, potassium, and omega-3 fatty acids; the researchers could compare the sodium intake of those who experienced heart attack, stroke, or death with those who did not.

The results are quite interesting: those who increased their sodium intake after the first yearly questionnaire were 72% more likely to experience a heart attack or stroke - and were twice as likely to die from them, while those who decreased it were 48% less likely to die from any cause.

What this means for you

The study authors note that using questionnaires to estimate sodium intake is not the most accurate way to measure the amount of salt in a person's diet. Nor can an observational study like this prove that one thing causes another, even though the authors took into account quite a number of variables. Even so, this adds yet more indirect evidence that those at higher risk of heart disease, like the participants in this study, would do well to keep their salt intake under 2,300 milligrams per day - the current Institute of Medicine recommendation.